natively
a ds flash cart flashes the gba rom to the 3-in-1 to play,
you can also burn the image to the cart so you can play that 1 rom without having to reflash it, it will work on other devices by its self like a gba player, gba without needing a ds flashcart everytime, if you want to change the rom you need to reinsert it into a ds/dsl and burn a new rom onto it

or you could just pay the extra $10-15 and get a stand alone ez-flash iv

natively
a ds flash cart flashes the gba rom to the 3-in-1 to play,
you can also burn the image to the cart so you can play that 1 rom without having to reflash it, it will work on other devices by its self like a gba player, gba without needing a ds flashcart everytime, if you want to change the rom you need to reinsert it into a ds/dsl and burn a new rom onto it

or you could just pay the extra $10-15 and get a stand alone ez-flash iv

It's Vi as in 5i not 6. It's pretty much the same as the V but w/ a rewritable boot strap to combat FW updates on the DSi/3DS. The EZ Flash team has not released an update since it was blocked many firmwares ago though when they went dormant.

W/ any luck the updates to the EZ4 is a sign of them coming out of hibernation.